Separation of hydrocarbons by permeation membrane



R. J. LEE

SEPARATION OF' HYDROCARBONS BY PERMEATION MEMBRANE Filed Oct. 29, 1954Aug. 2, 1960 s. w A i... .f

United States atent SEPARATION OF HYDRUCARBONS BY PERMEATION MEMBRANE`Robert Il. Lee, La Marque, Tex., assignor to The American Oil Company,a corporation of Texas Filed Oct. 29, 19154, Ser. No. 465,497

11 Claims. (Cl. 21o- 23) This invention relates to an improvement in theseparation of certain hydrocarbons from mixtures thereof with otherhydrocarbons by permeation through a nonporous membrane and it pertainsmore particularly to methods and means for increasing the rate ofpermeation of hydrocarbons through the membrane.

.It has been known that certain hydrocarbons could be (Serial Nos.443,893 and 443,895) are remarkably effective for separatinghydrocarbons according to type, and/ or molecular configuration, and/ orboiling point. Even with these improved non-porous membranes thepermeation rates of certain hydrocarbons may be two low for practicablepurposes; for example, normal heptane may be separated from a mixturethereof with isooctane but such separation is impracticable because ofthe very low permeation rate. An object of this invention is to providean improved method and means for increasing the permeation rate ofhydrocarbons through non-porous membranes which are capable ofseparating hydrocarbons according to type, and/or molecular conguration,and/or boiling point or molecular weight. A more specific object is toprovide a method and means for separating gasoline boiling rangehydrocarbon mixtures 'into fractions of high and low octane number,respectively, at such rates of permeation as to make theV processcommercially feasible. Other objects will be apparent in the course ofthe detailed description of the invention.

It has been discovered that when a hydrocarbon mixture is permeatedthrough a non-porous membrane in which certain of the hydrocarbonscontained in the mixture are more soluble than others, for the purposeof separating the mixture into different components, the rate ofpermeation can be greatly increased by contacting the membrane duringthe permeation process with a substituted hydrocarbon which is solublein and has solvent power for the membrane. The substitued hydrocarbon isan organic compound containing one or more atoms of an element such ashalogen, oxygen, sulfur, or nitrogen. These compounds are hereinafterreferred to as substituted hydrocarbon solvents. The substitutedhydrocarbon solvent is preferably of lower molecular weight and ispreferably a normally liquid compound. Organic halogen compounds such ascarbon tetrachloride, methylene chloride, ethylene dischloride, ethylenedibromide, dichloroethylene,

`trichloroethylene, trichlorouoroethylene, butyl chloride, l

bromohydrin, uorobenzene, and the like may be used as the substitutedhydrocarbon solvent. Oxygen-containing organic compounds includingalcohols such as ethanol, propanol, butanol and the like; ketones suchas acetone,

methyl ethyl ketone, methyl butyl fketone, cyclohexanone and the like;esters such as ethyl acetate, butyl acetate and the like; ethers such asethyl ether and the like; carboxylic acids such as propionic acid andthe like may be employed. Sulfur-containing organic compounds such asethyl mercaptan, propyl mercaptan, butyl mercaptan, diethyl sulfide,methyl butyl disulfide, butyl sulfone and the like may be used.Nitrogen-containing organic compounds such as nitropropane,nitrobenzene, acetonitrile, formamide, butyl amine, ethylene diamine,ethylene cyanohydrin, ethylthiocyanate and the like may also beernployed. It has been found that the substituted hydrocarbon solventscause an increase in the rateI of permeation of the hydrocarbons throughthe membrane without substantially altering the selectivity of themembrane for separating hydrocarbons. The substituted hydrocarbonsolvents such as have been listed are not all equally effective inimproving the permeation rates of hydrocarbons through the membrane. Theparticular substituted hydrocarbon solvent and the amount in which it isused will depend upon the improvement in the rate of permeation desired,the nature of the membrane, and the operating conditions employed in thepermeation process. The amount of the substituted hydrocarbon solventwhich may be employed may vary from about 1 to about 100% byconcentration based upon the total mixture of solvent and hydrocarbonswhich are present in the zone to which the solvent is added. As theconcentration of the solvent in the mixture which comes in contact with.the membrane is increased, the rate of permeation of the hydrocarbonsthrough the membrane is also increased.. Because the substitutedhydrocarbon solvent does have solvent power for the membrane it shouldnot be employed in concentrations which are so high as to dissolve orweaken the membrane to the point of rupture under the particularoperating conditions employed in the permeation process. The substitutedhydrocarbon solvent may contact the membrane either on the feed side,the permeate side or both sides. -It may contact the membrane while itis either in the liquid or vapor state. Preferably, the solvent is addedto the hydrocarbon mixture which is undergoing permeation.

`The method of separating materials by selective permeation throughnon-porous membranes has heretofore been described in the art. Copendingapplications Serial Nos. 443,893-4-5 set forth in detail the manner inwhich `hydrocarbons can be separated by permeation through non-porousmembranes. Briefly, a mixture of hydrocarbons is contacted with one side(feed side) of a thin (eg. 0.1 to 10 mils) non-porous membrane in whichcertain of the hydrocarbons contained in the feed mixture are moresoluble (preferentially permeatable) than others, a portion of thehydrocarbon mixture is' permeated through the membrane, and permeatedhydrocarbons which are enriched in those hydrocarbons more soluble inthemembrane are recovered from the opposite side (permeate side) of themembrane. In practice, each separation stage comprises a vessel which isdivided by the non-porous membrane into a feed zone for feedhydrocarbons and a permeate zone containing permeated hydrocarbons, eachzone having separate draw-offs. In order for permeation to occur it isessential that the concentra- `tion orfthe preferentially permeatablehydrocarbons in Vginal hydrocarbon components but having a higherconcentration (based upon total hydrocarbons permeated) 4of thepreferentially permeatable hydrocarbons than the concentration of theperferentially permeatable hydrocarbons present in the initialhydrocarbon mixture employed as feed. The remaining non-permeatedhydrocarbons present in the feed zone will have a lower concentration ofthe preferentially permeated hydrocarbons than was present in the feedand a lower concentration of these components than is present in themixture of permeated hydrocarbons. Conversely, the remainingnonpermeated .hydrocarbons will be enriched in those l-hydrocarbonswhich are less soluble .or less readily permeated through the membrane.

The permeation process .may be .conducted by contacting the .feedhydrocarbon 'mixture in either ,the liquid or vapor state with thenon-porous membrane and recovering .permeated .hydrocarbons from theopposite side of the membrane .in either the liquid or vapor state. rhepermeated hydrocarbons should not be allowed to accumulate within thepermeate zone to the .extent that the concentration of the more readilypermeatable hydrocarbons attain the lsame concentration in the medium ofthe permeate zone as they are present in the .feed zone. To facilitaterapid .permeation of hydrocarbons, the concentration of the .permeated.hydrocarbons at the surface of the ,membrane on the permeate sideshould be kept low by diluting permeated hydrocarbons in this zone witha diluent liquid .or gas .or by removing the permeated hydrocarbons. Thediluent employed to reduce the concentration of permeated hydrocarbonsin the permeate zonemay be liquid or gaseous, depending upon thephysical state of theperrneated hydrocarbons, and should be separablefrom the permeated hydrocarbons by distillation or `by other means.Examples of diluents are steam, air, Vbutano and the like. good methodof operating the permeation process consists of maintaining thehydrocarbon in the feed zone -in the liquid phase and removing permeatedhydrocarbons in the vapor phase from the permeate zone, using a .gaseoussweep to assist in removing the .permeated hydrocarbons from thepermeate surface of .the membrane.

The permeation process may be operated as a batch or .continuousoperation. When operating continuously,

.the -feed hydrocarbon mixture maybe continuously or intermittently.introduced .into the vfeed zone, and lnonpermeated and permeatedhydrocarbons are separately withdrawn `either continuously orintermittently from 'the feed and permeate zones respectively. The `rateof introduction of thefeed Vand the `removal of non-permeatedhydrocarbons .may be adjusted to provide the desired amounts Aofpermeated and non-permeated hydrocarbons. Of course, only aportion ofthe feed hydrocarbon mix- .ture should be permeated or .else noseparation :of the hydrocarbons will be obtained. .A number of permeatestages may be employed. Permeated or non-permeated hydrocarbons may be.recycled to the various zones. -In each zone the vmembrane may be usedin the form of sheeting or tubing or in any other manner whichpreferably provides a maximum amount of membrane surface to volumeratio.

In operating the permeation process the concentration of thepreferentially permeatable hydrocarbon should be lower in the permeatezone than it isin the feed zone. When operating with gaseous phases onboth sides .of the membrane this may conveniently be accomplished .bymaintaining a higher .absolute pressure vin the feed zone than in thepermeate zone. vPressure differentials of from mm. Hg to as high as 100p.s.i.g. orhigher maybe used, depending upon the strength of themembrane. vThe pressure in the particular Zone may thus vary fromsubatmospheric 'to superatmospheric. It is Vvpreferred, to

'Vmaintain the permeate zone at subatmospheric pressures feed zone andhydrocarbons in the permeate zone in the vapor phase, the feed zone maysuitably be maintained at atmospheric pressure or somewhat higher andthe permeate Zone may be maintained at a subatmospheric pressure so thatpermeated hydrocarbons are readily removed from the permeate side of themembrane and then removed from the permeate zone.

Thernernbrane employed is fnonfporous, Yi.e. free from holes or otherdefects which destroy a continuous surface. If the membrane has pinholes or the like which allow hydrocarbons to leak through, theselectivity of the permeation process -is reduced or eliminated. Themembrane `should l.be as thin as possible and yet retain sufficientstrength and stability to be useful inthe permeation process. Ordinarilyit may lbe yfrom about 0.1 to 10 mils in thickness. Higher rates -ofpermeation are obtained as the thickness of the membrane is decreased.Supports such as iine .mesh wire screen, porous sintered metals or.ceramic materials .may be used as backing or supporting means to assistin :minimizing the .chances of `rupturing the membrane -while yetemploying as thin a fmembrane as possible. The membrane must .be one inwhich certain hydrocarbons are more soluble than others. Examples .of.such .selective Vmembranes are those comprised of natural or syntheticrubber such Vas gum rubber, chloroprene or neoprene rubber, vinylpolymers such as styrene polymer, polyisobutylene whose constituentshave an average molecular weight 'higher than about 10,000, .certaincellulose esters such as are described vin detail on page 1, line y22,to page 4, ,line 17, of copending application Serial No. 443,894 AandAclaimed therein for ,such purpose, and certain .cellulose ethers suchas are described in detail .on lpage l, line 22, to page 4, line 19, o fcopending application Serial No. 443,893 and claimed therein .for thepurpose of yselectively .permeating certain hydrocarbons. The preferredmembrane is comprised of ethyl cellulose having .an ethoxyl content4between 40 yto 47% by Weight. A membrane comprised of celluloseacetate-butyrate having an acetyl rcontent of about 5 to 15% anda'butyrylcontent of 35 to 60% may also be`used although lower permeationrates are obtained.

The membranes described supra arenot necessarily equiv- -V alent to eachother .in regard to rate `of permeation and selectivity, and .it is onlynecessary that the vmembrane be capable of selective permeation.

vThe hydrocarbon mixture which may be `employed as thefeed .to v.thepermeation process .may Vbe a natural or synthetic .mixture .ofhydrocarbons. ,Minor amounts .of `impurities .may be present therein..Hydrocarbon mixtures can be .separated into fractions .having diteringconcentrations of hydrocarbons of a :particular type, i.e. aromatics,unsaturated and saturated hydrocarbons. In general, Athe various typesof .hydrocarbons permeate more rapidly `through the membrane -in thevfollowing order: saturated hydrocarbons, .unsaturated hydrocarbons ,andaromatic hydrocarbons. Hydrocarbons can be separated according tothestructural configuration of the lhydrocarbon molecules-present .in thefeed. r-Ihus hydrocarbons -can .be separated according to theirmolecular configuration into fractions 'enriched in cyclic,:branchedchain, and straight-chain hydrocarbons, the particularcongurationpermeating more rapidly through the membrane in Vthe -orderlisted when 'the hydrocarbons Vhave the same numberofcarbonatoms(.fifofthesame boiling point theorder of vincreasing permeationrates isbranchedchain, cyclic-chain 4and straight-chain). Thernorehlghlybranched :hydrocarbons permeate less readily thanvdo .the hydrocarbonshaving a lesser Ydegree lof branching, and consequently .a separationbetween :them vcan be .eiected .Separation .according -to JmolecularVconfiguration :is particularly 'effective -when the hydrocarbons `are`of thesame .type such aseither olens or parafns ,A separationcan ialso:be ymade between hydrocarbons .according to v,their molecular weight,the lower .molecular weight h ydrocarhonrsenerrally.Prmeatnsthrough.themembrane more vration is most strikingly performed upon a feed which isa narrow boiling mixture of hydrocarbons, e.g. boiling within a range ofaboutV 30 C. or less, which preferably boils within the gasoline boilingrange. When employ- `ing the substituted hydrocarbon solvent yfor themembrane to improve the rate of permeation, greater improvements in thepermeation rate can be obtained when the :feed is lean or issubstantially free of aromatic hydro carbons and unsaturatedhydrocarbons. A preferred feed stock is comprised essentially ofsaturated hydrocarbons boiling within the gasoline boiling range andhaving a boiling range of about 30 C. n

By contacting the membrane during the permeation step with a substitutedhydrocarbon solvent for the membrane the rate of permeation can beincreased. In certain instances the use of a substituted hydrocarbonsolvent improves the selectively of the separation which is obtained,whereas in other instances the selectivity is substantially not alteredor only slightly diminished. 'Ihe substituted hydrocarbon solvents areorganic compounds `which are soluble in and have solvent power for themembrane and which contain at least one atom such as a halogen, oxygen,sulfur, or nitrogen atom. They may be considered to be hydrocarbons intowhich an element, such as have been listed supra, is introduced, thusVthe term substituted hydrocarbon solvent. Organic halogen compoundswhich are soluble in the particular membrane and have solvent powertherefore may be used as the substituted hydrocarbon solvent. Compoundssuch as carbon tetrachloride, methylene chloride, ethylene dichloride,ethylene dibromide, dichloroethylene, trichloroethylene,trichloroiluoroethylene, butyl chloride, bromohydrins, fluorobenzene,and the like may be employed. Oxygen-containing organic compounds whichlikewise dissolve in the membrane and exert a solvent power or action onthe membrane may be employed. For example, alcohols such as ethanol,propanol, butanol and the like; ketones such as acetone, methyl ethylketone, methyl butyl ketone, cyclohexanone, Iacetonyl acetone, mesityloxide and the like; esters such as ethyl acetate, butyl acetate and thelike; ethers such as ethyl ether, tetrahydrofuran, dioxane, methylCellosolve and the like; carboxylic acids such as propionic acid and thelike may be ernployed. Sulfur-containing organic compounds, e.g.mercaptans such as ethyl mercaptan, propyl mercaptan, butyl .compoundswhich dissolve in the membrane and have a solvent power therefore may beused. The various substituted hydrocarbon solvents are not equal intheir effect in improving permeation rates of the hydrocarbons. Theireffectiveness may differ remarkably when selective membranes of diderentcomposition are used. Among the preferred substituted hydrocarbonsolvents are the oxygen-containing organic compounds, especially methylethyl ketone.

When the substituted hydrocarbon solvent comes in contact with themembrane it enables hydrocarbons to permeate the membrane more readily.The substituted hydrocarbon solvent may be introduced into the permeartion step by adding it to the feed zone or the permeate zone in either aliquid or gaseous condition. Ordinarily and substituted hydrocarbonsolvent.

the physical stateiof the substituted hydrocarbon solvent should be thesame as for the hydrocarbons which are -present in the zone to which itis added, it is preferred to introduce the substituted hydrocarbonsolvent into the feed zone either by separate introduction orintroduction along with the feed hydrocarbon mixture. A desirable methodof operating consists of maintaining the feed hy- ,drocarbon mixture andthe added substituted hydrocar- Vibon solvent in the liquid phase in thefeed zone and recause the membrane to weaken to the point of ruptureunder the. operating conditions employed in the permea- Ation process.The amount of solvent used will vary with the increase in permeationdesired, the composition of the membrane employed, the particularsolvent used, and

the operating conditions in the permeation process, i.e., thecomposition of the hydrocarbons, the physical state of the feed and thepermeate, the temperature, the pressure, and the pressure differential.'I-n general, the amount added may be such that the concentration of thesubstituted hydrocarbon solvent (based upon the total amount of solventand hydrocarbons present in the particular zone to which the substitutedhydrocarbon solvent is added) may vary from 1 to 100; the higher theconcentration of the solvent, the greater is lthe increase in the rateof permeation of the hydrocarbons. Under a given set of permeationoperating conditions as the concentration of the solvent is increasedthe membrane begins to soften slowly until it is severely softened,thenl gels, and finally dissolves in the mixture of hydrocarbons As thesubstituted hydrocarbon solvent `dissolves within the membrane, it canfrequently be observed that the membrane swells. At this stage it isbelieved the major elfect of the solvent for increasing the permeationrate of hydrocarbons through the membrane has been achieved. A furtherincrease in the concentration of the solvent will greatly increase thedanger of weakening the membrane to the point of rupture. Whensubstituted hydrocarbon solvent in the vapor phase contacts the membraneits solvent power for the membrane is not as great as it` would be if itwere in the liquid phase. Consequently, the concentration of the vaporsof the substituted hydrocarbon solvent in contact with the membrane maybe greater without causing the membrane to weaken to the point ofrupture.

Theoperating temperature of the permeation process also has a bearingupon the stability of the membrane which is in contact with thesubstituted hydrocarbon solvent. It may vary between 0 to 250 C.depending, of course, upon the particular membrane used, the particularsubstituted hydrocarbon solvent employed, the concentration of thesolvent in contact with the membrane, and the composition of thehydrocarbons in contact with the membrane. In general, as thetemperature is increased the rate of permeation of hydrocarbons throughthe membrane is also increased, but the stability of the membrane isdecreased and there is a danger of the membrane rupturing. Somewhatlower temperatures are therefor used when operating the permeationprocess in the presence of the substituted hydrocarbon solvent thanwould be used when operating in its absence.

The various concentrations of the particular substituted hydrocarbonsolvent and the permeation temperatures which may be fused can bedetermined by a few simple tests. While maintaining the temperatureconstant, a sample of the particular membrane to be used is mersed in abeaker containing the hydrocarbon mixture atomes# .brane'is observed. Asthe membrane swells lor 'becomes :somewhat sottend, the .concentrationof the substituted .hydrocarbon solvent lin the hydrocarbon mixture is`then `noted. This ,concentration ispreferably no texceeded in theliquid :mixture employed `as the permeation chargeat the 'particulartemperature at which .the test was conducted, forto do sowould causeserious .danger tof lthe .membrane rupturing. The `effect:of-temperature on the meated hydrocarbons and substituted hydrocarbon'sol- 3 vent are contained therein inthe liquid phase. Somewhat higherconcentrations of substituted hydrocarbon `solvent and/or highertemperatures may be employed if the mixture of hydrocarbons and solventcontacts .the membrane While in the vapor state. Simple tests of the.type described maybe vused to determine these concen- Vftrations andtemperatures while Imaintaining the mixture of hydrocarbons landsubstituted hydrocarbon solvent in ith-e vapor state in Acontact withthe membrane.

When employing a membane comprised of cellulose acetate -butyrate(having .a butyryl content o-f 40-5070 and Aan acetyl content of5-`l5%), it is preferred to vernvploy .methyl ethyl ketoneas thesubstituted hydrocarbon solvent. When operating in accordance with thepreferred `method of maintaining feed hydrocarbons and ladded solvent inthe liquid phase andremoving permeated hydrocarbons .and .permeatedsolvent from fthe opposite lside lof the membrane in the vapor phase, .a.permeation .temperature of to 90 C. and a concentration o-f up to byvolume of added methyl ethyl ketone in the feed hydrocarbons ispreferred. When the membrane is comprised lof a cellulose 'ether such as.ethyl cellulose having Tan ethoxyl 'content of 4 01to 47%, thepreferred substituted hydrocarbon solvent is acetone. It may be added tothe :feed hydrocarbons in up to about 5% by volume, andthe permeationprocess operated at a temperature of about 0 to 80 C. When a syntheticrubbertype A'membrane is employed, ethylene dichloride is the preferredsubstituted hydrocarbon solvent. 4it `is added 'to the -feedinconcentrations up to about 10% oyvo-lume,

and the .permeation process is operated at a temperature of about 0 to1009 C.

The substitutedhydrocarbon solvent used is preferably one whichis-'easilysepar-ablefrom the feed and permeated hydrocarbons. Itpreferably `has a boiling point differing extractedV from .the rfeed orpermeated hydrocarbons by washing -with -water and then .distilling ,thesolvent Afrom .the aqueous 'solution thereof. Recovered substitutedhydrocarbon sol'cent .can then b e reused. In multi-stage operations.wherein either the permeated hydrocarbons or the non-permeatedhydrocarbons are sent to later or eariler .permeation stages as aportionof the .feed, it may be desirable to allow .the substitutedhydrocarbon vsolvent to in thefraction which is sent to the later orearlier permeationstage; provided the proper concentramembrane ,can ,bemaintained in the desired operating range.

4A number .of experiments were performed 'which Idemonstrate theeffectiveness :of the substituted hydrocar- :honzsolventin improving therateofpermeationofa mix- .ture of saturated hydrocarbons .through4nonporous selective membranes. vIn these experiments a -permeation y.c,e1'1 .whose construction and manner'of ,operation vis-de scribed indetail in copending lapplication Serial No. .443,894 (page 15., line 9,topage :16, line was used. The apparatus vcomprised achamber in whichwas sus- .pendedta-boxflikemembrane holder. The-totalamountoffmembranesurface eective for permeation was 22 square tps inches. 'T [he.membrane holder had 5 Aopen faces which were tightly :covered (leakjproof) with the membrane. The .chamber functioned asthefeed .zone andit had profvisionsfor introducing and -Withdrawingthe feed and-non--permeated hydrocarbons, respe ctively. The interior .of the box-like`membrane holder functioned as (the permeate zone and is connected witha line ,for withdrawing permeated .hydrocarbons andrecovering the same.The permeate zone .wassealedtoff from the-feed zone so-.that nohydrocarbons -could enter the permeate Izone .except .by permeatingthemembrane. Provisions were made `in Ithe .apparatus forsweeping thevpermeate surface ofthe-mem- .brane with .answeep gas. .Pumps were soassociated with .the .apparatus that .any range of pressures fromsubatmos- .-pheric to :atmospheric could .be maintained in either therfeed zone or the,permeate.zone. Y

The permeation cell was employed to .separate methylv cyclohexane froman .egual volume mixture :of methylcyclohexane with visooctane .in the.absence Aand Ain the presence ,of methyl ethylfketoneas .thesubstitutedhydrocarbon membrane solvent. When using methyl 4ethylkevtone it .was contained in the `feed hydrocarbonmixture at laconcentration of 5% by volume. ployed was Tennessee .Eastman vCelluloseAB-SOO- S (cellulose .acetate b -utyrate having a .butyryl content of.about 48% and an acetyl .content of ,about 7%.) having a thicknessrofabout 1.5 mils. The hydrocarbons and substituted hydrocarbon solventwere maintained in the liquid lphase in the feed zone and in the vapor,phase in the lpermeate zone. Permeatio n temperatures of 52 C. vand '82C. were used. A pressure differential .of about 4.00 mm. Hg wasmaintained across the .rnembrane, theabs o lutepressurein the permeatezone-being about mm- Hg and the absolute pressure in the feed zonebeingabout 435 mm. yApproximately 1.40.() ml. of hydrocarbons were introducedas the feed. The various mixtures -employed in .each run were permeatedthrough the membrane until approximately `a steady rate of permeationhad been attained, usually between l to 6 hours. The amount Aofpermeated hydrocarbons recovered varied vfrom about 6 to 36 m1,depending upon the time and .rate ofpermeation. Table I below sets forthdata which were obtained.

Table 1 Permeate Composition, Perme- Tern- Charge -Volu'me Percent ationpera- .Gomposi- Hotel Run ture, tion.`Vol v Solvent- No. .G. f ume:Percent Total vSolvent- .Free Basis Free Basis Basis l MCIL..- 85 MGEL.-.85 lVICH. 15 Isooct.. i o" 73 MGEL.-- Z 27 Isooct .l s 7s MolT,. 3 u 475 non sa on 22 Isooctm" 1.6 l M N 4.----; fs2= unsono... :13 Isooctm..75 LCH-m- 5.0

{5.0 Solvent.. 48 Solvent... 25 1500m-"- 1 Gallons/hiz/LOQO sq. ft. ofmembrane surface.

,It can be seen from the data ithat addition of 5% tby .tion .of.solvent in vthe hydrocarbons in .contact with the 5ni volume of methyl'ethyl .ketone tothe feed hydrocarbon The membrane ernyfeed zone wasapproximately 200 ml. in all runs.

mixture causes the rate of permeation of the hydrocar- 'bons to increaseto about 3-fo1d, with only a small drop in selectivity. An attempt tcemploy methyl ethyl ketone in a concentration of by volume based uponthe total mixture of substituted hydrocarbon solvent and hydrocarbons inthe feed zone resulted in rupturing of the membrane. It should be notedalso that the higher hydrocarbon permeation rates were obtained at thehigher temper-atures. Thus, one should operate with as high aconcentration and at as high a temperature as is possible withoutcausing the membrane to become so weakened that it is easily ruptured.

An additional series of runs were per-formed using a permeationapparatus of the same type but of smaller size (1.8 sq. in. of membranesurface) than was employed 4in obtaining the dataset forth in Table I. ATennessee Eastman Cellulose A-B-38l (cellulose acetate butyrate having abutyryl content of about 38% and an acetyl content of about 13%)membrane of about 1.1 mils thickness was used. In all the runs the yfeedhydrocarbons, which were an equal volume mixture of methylcyclohexaneand isooctane, and the added substituted hydrocarbon solvent `contactedthe membrane while in the vapor state.

n-Butanol and methyl ethyl ketone were employed as the substitutedhydrocarbon solvents. Hydrocarbons and substituted hydrocarbon solventwhich permeated the membrane were maintained in the vapor state in thepermeate zone. Permeation temperatures of 45 to 56 C. were used. Apressure differential of about 157 mm. Hg was maintained across themembrane, the absolute pressure in the feed zone being about 166 to 172mm. HgA and the absolute pressure in the permeate zone being about 10-14mm. Hg. The volume of charge to the The volume of permeated hydrocarbonsranged between 0.3 to ml. Air was used to sweep the permeate surface ofthe membrane. Table II below sets forth the data which were obtained.

1 Gallons/hr./1,000 sq. it. of membrane surface. 2 Small amount did notpermit analysis.

It should be noted from runs 6 and 7 that a four-fold and fourteen-foldincrease, respectively, in the rate of hydrocarbon permeation wasobtained by use of the substituted hydrocarbon solvent. An increase ofsuch magnitude with no great loss in the selectivity of separation makesthe separation of hydrocarbons by selective permeation throughnon-porous membranes a much more attractive process.

Another series of experiments were conducted employing an apparatus of adifferent type. In these experiments a coil of neoprene tubing having aninside diameter of one-fourth inch and a thickness of 62 mils was usedas the membrane. A portion of the coil was `disposed within a containerfor the feed hydrocarbon mixture. The interior of the tubing served asthe permeate zone, and the solvent was continuously circulatedtherethrough while in the liquid state. One end of the tubing wasconnected to `a reservoir containing substituted hydrocarbon solvent andthe other end of the ltubing had attached to ita pumpfor circulating thesolvent-from the reservoir through the .tubing and back.

to the solvent reservoir.` The container fr the `feed hydrocarbonmixture was charged with about 1000 m1. of the feed hydrocarbon mixture.Approximately 205 om. of tubing was in contact with the hydrocarboncharge. 'I'his was equivalent to approximately 80.6 square inches ofmembrane surface in contact with the feed hydrocarbon. The volume ofsubstituted hydrocarbon solvent which was continuously recirculatedthrough thel permeate zone was about 500` ml. Before the data for eachrun were collected, a break-in run of about 2 hours was made so that therate of permeation hadbecome approximately steady. The data for each runwere then collected. The concentration of permeated hydrocarbons whichaccumulated in the recirculatingA substituted hydrocarbon solvent duringthe course of each run varied from about 3 to 10%. The permeatedhydrocarbons were then separated from the solvent, measured, `andanalyzed. When methanol and acetone were employed as the solvent, thesolvent was separated from the permeated thydrocarbons by addingapproximately 1 to 2 volumes of water thereto and separating thehydrocarbon phase from the aqueous phase. In the experiments whereincetane and decane (nonsolvents) were employed in place of thesubstituted hy- Vdrocafrbon solvents as standards for comparison, thepermeated hydrocarbons were separated therefrom by distillation. In theruns, for which data are shown in 'Table III below, the feed consistedof a' liquid mixture of about 66% toluene and 34% n-heptane. Thetemperature of permeation was maintained at about 35 C. The resultsobtained are shown below in Table III.

1 Gallons/hr-ll000 sq. ft. of membrane surface.

It is apparent from the above data that by contacting the membraneduring the permeation process with a substituted hydrocarbon solvent,the rate of permeation of hydrocarbons through the membrane isincreased. It should be further noted that the selectivity of themembrane for permeating toluene is increased. 'Such improvements renderthe separation of hydrocarbons by the permeation process much moreattractive from an economic viewpoint.

The invention will be more clearly understood by reference to thefollowing specific example illustrated in the annexed drawing whichforms a part of this specification and shows in schematic form oneembodiment of `the process of this'invention for separating a mixturecomprised of saturated hydrocarbons of differing rnolecularconfiguration into fractions enriched in hydrocarbons of a particularmolecular configuration.

The feed employed in this illustration is comprised essentially of anequal volume mixture of n-heptane and isooctane. It is passed fromsource 11 at a temperature of about 701 C. by way of valved line 12 intothe feed `zone 13 ofl the Ifirst permeation stage. The substitutedhydrocarbon solvent for the membrane, in this illustration acetone, ispassed from source .14 by way of valved line 16 land at a temperature ofabout 70 C. into valved line v12. wherein it mingles with the saturatedhydrocarbon :feed and passes into feed zone 13 of the rst Vpermea'tionstage. VAs illustrated diagrammatically herein, the first permeationstageconsists of a vessel 17 which is divided by a non-porous membrane18 .toform -two vertical sections, one being the feed .zone 13 and theother being -the permeate zone .19. The non-porous membrane Iwhich isemployed in each Vof the three stages .ofthe .embodiment described iscomprised yof Hercules Ethoeel G-lOO which is an ethyl cellulosemembrane .having an ethoxyl content .of 44.5-45.5%. The thickness ofVthe membrane .fused is 1.5 .mils. Although .not illustrated, eachpermeation stage fmay .becomprisedoffa fgreat .number of individualpermeation cells, each cell consisting of a .feed zone lseparated `by anon-porous membrane from a permeate zone. The .particular form ofapparatus used in each permeation .stage vmay be widely varied andconstitutes no particular part :of this invention. Likewise, the.membrane vmay y.be I.one which is supported insome mannerzto-.diminishthe possibility of membrane rupture. The kconcentration of acetone inthe mixture of normal heptane and isooctane in -each stage is maintainedat .about 5% `b y -`volume. The feed zones of each permeation Vstage aremaintained .under a .superatmospheric pressure of 3() j.p.s.i.a Under.these conditions,-the hydrocarbons and acetone in the :feed .zone arelmaintained in the liquid phase. The pressure .maintained in thepermeatefzone of -each vpermeation .stage is about 1700 mm. Hg abs.Eorpurposeoficlarity, .the numerous pumps and other vequipment necessary.to .maintain such conditions of pressure .are -not ,detailed fherein.The mixturecf acetone,-normaltheptane and iso- .octane iis introducedinto the feed zoneeof each permeation stage at a rate suc-h that.approximately 7% .of the introducedhydrocarbons permeate the membrane.and about 1/3 of the total hdrocarbons fed into each feed zone areremoved as non-permeated hydrocarbons. 4Greateror vlesser amounts of theintroduced hydrocarbons may be allowed to permeate the membrane in :eachstage. Although not critically necessary, a sweep ygas is employed inthis illust-ration to assist in removing permeated vhydrocarbons andsolvent from Ythe permeate side of the membrane. Butanefrom source 21 isadmitted as a gas'by way of `valved line 22 into permeate zone 19 forthis purpose.

The non-permeated hydrocarbons and acetone Vare removed from the `firststage and passed by way of valved line 23 into feed zone 24 of thesecond permeation stage. This non-permeated `fraction is enriched infisooctane and is depleted in normal heptane. It is also 'depleted inacetone. The vpermeated hydrocarbonsand acetone together with the sweepgas butane yare removed from permeate zone 19 and passed by'way of line26 :to cooler 27. The mixture isthen passed by way of line 2S intofractionator 'S29 wherein an overhead gaseous "butane stream isseparated Vand `recycled iby way of'line 31 for use as the -sweepgas inthe'various Vpermeate zones. Acetone which tends to concentrate in thepermeate fraction is also separated in fractionator 29 and is Vremovedas a side stream by way of line 32. 1t is lthen passed to `condenser :33and recycled by way of line 34 as the solvent for the membrane used inthe feed zones of the various permeation stages. Preferably the solventrecovered 'from "the `fraction of the permeate is 'recycled to the feedlzone ofthe same permeation stage. Alliquid bottoms 'fraction is removedfrom fractionator 29 by way of line 36. VfThis liquid mixture ispartially -concentrated n-heptane containing a small proportion ofisooctane. Additional acetone is added to the mixture .in-valved line'2.3 sothat the concentration of acetone is by volume inthethree-component hydrocarbon -mix- Vture in feed zone 2.4 of thesecond permeation stage. The acetone introduced may be that introducedfrom source 14 .by means not shown, for as in this illustration 'it maybe acetone which has ,been recoveredfrom the permeate Vfraction ofth'e'frst permeationjsta'ge Y'which is `being cycled by way of line 34.Valved line 3734's employed to carry the additional amount Lof vacetone`vrneeded from `line 3dto line .ation zone .49 Vof the Vthird permeationstage; non-permeated hydrocarbons have become enriched in .ate zone 41-of the .second permeation stage. .per- `meated hydrocarbons .andacetone `together with the sweep Agas butane are removed from permeate.zone 41 and passed by way of line 42 .to cooler 43. The mixture iscompressed by means not shown and passed by way of line 44 into`fractionator 45. Gaseous butane is recovered overhead fromf-ractionator 45 and is passed by way of line 46 into the main butane`recycling .line 31. Acetone is removed from fractionator 45 as asidestream and is passed .by -way of line 47 to acetonerecycling line 34. Abottoms .fraction consisting 4of normal heptane and isooctane isrecovered from fractionator 45 and is passed by way .ofline 48 into linev1,2 for the recovery of further amounts of the isooctane stillremaining. Certain permeated hydrocarbons, Vafter romoval of sweep gasbutane, can lbe recycled `as aportion of the feed to the feed zone ofprevious `permeation stages if ythe concentration of the .hydrocarbonsinthe .permeate fraction is approximately the same as that entering theYfeed zone of the V,earlier permeation stage. This enables a morecomplete recovery of the `desired isooctane.

The non-permeated hydrocarbons and acetone re- .rnoved `from feed zone2.4- of the second -permeation stage are passed by way vof valved line`33 into `.perme- :These isooctane and depleted Ain .normal heptane andacetone. Additional acetone is added to valvedline 38 to increase theconcentration of acetonein the hydrocarbons in feed Y zone 49 -to about5%. As illustrated here, Athe acetone is admitted from line 34, whichcarries recycled acetone, by way of -valved .line .51 `into -val-vedline .38. .Bntane sweep gas is vadmitted from line 31 by way of valvedline .52 into permeate zone 55 of the third `permeation stage. Thepermeated hydrocarbons and acetone V4together `with Vbutane are .removedfrom Vpermeate lzone 53 and passed by way of line 54 into cooler l56.The cooled .mixture is then compressed .and sent by way of line 57 tofractionator 5S. Gaseous butane is removed overhead from .fractionator58 and passed by way of line 59 into Ithe .butane recycling 31 forreuse. Acetone is removed from fractionator 5S as a side stream and ispassed by way of line 6% to acetone recycling line 34. This liquidhydrocarbon mixture .is Yenriched in .isooctane and constitutes avaluable stream for recycle tothe Vfeed zone of the second permeationstage as illustrated herein. The non-permeated hydrocarbons are removedfrom feed zone 49 and passed by -way ofvalved .line 162 intofractionator 63. This non-permeated fraction .consists mostly ofisooctane with only a small amount of acetone and normal heptane.Acetone is separated from this 1mixture and taken overhead and passed byway of line 64 into acetone recycling line .34. A highly concentratedisooctane fraction containing only .a =very small amount of normalheptane is recovered from fractionator 63 and is removed as a liquidbottoms :iproduct by 'way of line v66. This highly .concentratedrisooctane fraction is an excellent motor fuel .or rmotorafuel blendingagent. The concentrated normal 'heptane :recovered from the system `by.way ,of line 36 'provides ,a suitable feed material for aromatizationto a `higher octane number Amotor fuel. v-By means of `this inver;d tiona highly .eficient and rapid selective permeationo hydrocarbons `isVobtainable. Y

The yuse 'of hydrocarbon solvent `such .as 4aromatic ,or

.unsaturated .hydrocarbons for improving -the .permeationz rateof-saturated ."hydrocarbons throughv selective nonporous membranes isthe subject matter of cop'ending application Serial No. 465,495, tiledon even ydate herewith.

Thus having described the invention what is claimed 1s:

1. In the process of separating a mixture of at least two hydrocarbonsby introducing said mixture into the feed zone of a permeation apparatuscomprised of a feed zone which is sealed from a permeate zone by a thinplastic membrane in which one of the hydrocarbons contained in saidhydrocarbon mixture is more soluble than others, in Which process aportion of the mixture of hydrocarbons is permeated through saidmembrane into the permeate zone and the permeated portion is withdrawnfrom said permeate zone and a non-permeated portion is withdrawn fromthe feed zone, said permeated portion being enriched in the hydrocarbonwhich is more soluble in the membrane, the improvement which comprisesincreasing the rate of permeation of hydrocarbons through the membraneby introducing into the feed zone `a substituted hydrocarbon solvent forsaid membrane, the introduced solvent being in addition to that normallyoccurring with the feed hydrocarbons, and carrying out the permeation inthe presence of said substituted hydrocarbon solvent in an amount of the1atter suicient to substantially increase the rate of per meation ofhydrocarbons through said permeation membrane, and thereafter separatingsaid substituted hydrocarbon solvent from the permeated hydrocarbons.

2. The process of claim 1 wherein said substituted hydrocarbon solventis added together with the mixture of hydrocarbons introduced into thefeed zone.

3. The process of claim 1 wherein the substituted hydrocarbon solvent isseparable by distillation from the hydrocarbons contained in saidmixture of hydrocarbons.

4. The process of claim 1 wherein the mixture of hydrocarbons is anarrow boiling mixture.

5. The process of claim 1 wherein the mixture of hydrocarbons iscomprised essentially of saturated hydrocarbons.

6. The process of claim 1 wherein the mixture of hydrocarbons andsubstituted hydrocarbon solvent is maintained in the liquid state in thefeed zone and the permeated portion is withdrawn in the vapor state fromthe permeate zone.

7. The process of claim 1 wherein the mixture of v 10. The process ofclaim l wherein said substituted hydrocarbon solvent is ethylenedichloride.

11. In the process of separating a mixture of at least two hydrocarbons,said mixture being a narrow boiling mixture which boils Within thegasoline boiling range and is substantially free of aromatichydrocarbons by introducing said mixture into the feed zone of apermeation apparatus comprised of a feed zone which is sealed from apermeate zone by a t-hin plastic membrane in which one of thehydrocarbons contained in said hydrocarbon mixture is more soluble thanothers, in which process the mixture of hydrocarbons in the feed zone ismaintained in the liquid state and a. portion of the mixture ofhydrocarbons is permeated through said membrane into the permeate zoneand. the permeated portion is Withdrawn in the vapor state from saidperJ meate zone and a non-permeated portion is Withdrawn from the feedzone, said permeated portion being enriched in the hydrocarbon which ismore soluble in the membrane, the improvement which comprises increasingthe rate of permeation of hydrocarbons through the membrane byintroducing into the feed zone a liquid substituted hydrocarbon solventfor said membrane, the introduced solvent being in addition to thatnormally occurring with the fed hydrocarbons, and carrying out thepermeation in the presence of said substituted hydrocarbon solvent in anamount of the latter suicient to substantially increase the rate ofpermeation of hydrocarbons through said permeation membrane, andthereafter separating said substituted hydrocarbon solvent bydistillation from the permeated hydrocarbons.

References Cited in the le of this patent UNITED STATES PATENTS -ModernPlastics for June 1950, pp. 97, 9S, 100, 102, -152, 154, 156, 158(article by V. L. Simnil and A. Herschberger).

Modern Plastics for .lune 1951, page 107.

Technique of Organic Chemistry, vol. III, Pt. I, Separation andPurification, by Arnold Weissberger. First Ed., published byInterscience Publishers, 1956, pp. 41-47.

1. IN THE PROCESS OF SEPARATING A MIXTURE OF AT LEAST HYDROCARBONS BYINTRODUCING SAID MIXTURE INTO THE FEED ZONE OF A PERMEATION APPARATUSCOMPRISED OF A FEED ZONE WHICH IS SEALED FROM A PERMEATE ZONE BY A THINPLASTIC MEMBRANE IN WHICH ONE OF THE HYDROCARBONS CONTAINED IN SAIDHYDROCARBON MIXTURE IS MORE SOLUBLE THAN OTHERS, IN WHICH PROCESS APORTION OF THE MIXTURE OF HYDROCARBONS IS PERMEATED THROUGH SAIDMEMBRANE INTO THE PERMEATE ZONE AND THE PERMEATED PORTION IS WITHDRAWNFROM SAID PERMEATE ZONE AND A NON-PERMEATED PORTION IS WITHDRAWN FROMTHE FEED ZONE, SAID PERMEATED PORTION BEING ENRICHED IN THE HYDROCARBONWHICH IS MORE SOLUBLE IN THE MEMBRANE, THE IMPROVEMENT WHICH